Various relatively small motorized vehicles, such as snowmobiles, all-terrain vehicles (ATV's), tractors, motor scooters, go-carts and golf carts use an endless belt type continuously variable transmission (CVT). Variable transmissions include a variable-input drive/driving/primary pulley or clutch and an output driven/secondary pulley or clutch. The drive pulley is connected to the crankshaft of the engine. The driving pulley is also called the input pulley because it is where the energy from the engine enters the transmission. The second pulley is called the driven pulley because the first pulley turns it. As an output pulley, the driven pulley transfers energy to the driveshaft of the track drive. Each pulley is composed of a fixed sheave or pulley half that is fixed in the axial direction, and a movable sheave or pulley half, which is movable in the axial direction. A high strength metal or rubber belt, such as a V-belt, joins the drive pulley and the driven pulley and rides in the groove between the two sheaves. When the two sheaves of the pulley are far apart, the belt rides lower in the groove, and the radius of the belt loop going around the pulley gets smaller. When the sheaves are close together, the belt rides higher in the groove, and the radius of the belt loop going around the pulley gets larger.
Thus, the effective radius of both the primary and the secondary pulley is variable. The ratio of the primary pulley radius to the secondary pulley radius determines the ratio of engine rotational speed to the secondary shaft rate of rotation. When the primary clutch radius is smaller than the secondary clutch radius, the secondary shaft turns at a rate that is slower than the engine speed, resulting in a relatively low vehicle speed. As the ratio of the primary and the secondary clutch radius approaches 1:1, the secondary shaft speed will be approximately equal to the engine or crankshaft speed. As the primary pulley radius becomes greater than the radius of the secondary clutch, an overdrive condition exists in which the secondary shaft turns at a greater rate than the engine crankshaft. CVT's may use hydraulic pressure, centrifugal force or spring pressure to create the force necessary to adjust the pulley halves.
When one pulley increases its radius, the other decreases its radius to keep the belt tight. As the two pulleys change their radii relative to one another, they create an infinite number of gear ratios—from low to high and everything in between. For example, when the pitch radius is small on the driving pulley and large on the driven pulley, then the rotational speed of the driven pulley decreases, resulting in a lower gear. When the pitch radius is large on the driving pulley and small on the driven pulley, then the rotational speed of the driven pulley increases, resulting in a higher gear. Thus, in theory, a CVT has an infinite number of gears through which it can run at any time, at any engine speed or at any vehicle speed.
These variable transmissions are equipped with a speed or revolution per minute (RPM) responsive mechanism associated with the drive pulley and a torque responsive mechanism associated with the driven pulley. Therefore, the drive pulley and the driven pulley continuously vary the shift ratio in relation to the drive speed and the driven torque.
The primary clutch is connected to the power source and in theory has the job of maintaining the engine's RPM at a value where the most power is being produced by the engine. The primary clutch may also control engagement and disengagement of the engine from the load in order to stop and start vehicle movement. In the case of a snowmobile, the secondary or driven clutch is connected to the load through a jackshaft, gears, chain and track and functions to change the ratio of the two clutches as the load varies. This function is performed by a torque sensing helix or the like, which is typically considered part of the secondary clutch.
Prior art clutches are typically of the cam arm and roller type or comprises a sliding block. The cam arm and roller type clutches are prone to premature wear of the cam arm and the roller, which results in earlier and more frequent replacement. The calibration of force on the belt can also produce excessive belt heat, which results in belt failure.
With respect to the sliding block type of clutch, these types of clutches have calibration characteristics that prevent the clutch from producing desired performance. As both types of clutches used today have varying functional limitations, calibrating a clutch for desired performance often results in undesired inherited clutch characteristics.
Drive clutch systems of the prior art lack durability due to wear and tear caused by radial movement from the engine. Engines develop instantaneous RPM changes during operation and produce torsional vibration and varying inputs. Radial movement, which for purposes of this disclosure, comprises torsional vibration, instantaneous RPM and varying input, causes roller wear and surface deterioration of the cam arms as the result of the arms moving across the direction of support of the cam arm bearing and the roller bearing. Prior art methods of minimizing this wear including improving machining tolerance, precision improvements to assembly practices and using higher quality material, all of which increase the costs associated with the clutch device.